1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a magnetic recording medium which enables high-density recording by virtue of a high coercive force and, particularly, can suppress an increase in noise or preferably can reduce noise, and also can improve a reproducing output and S/N ratio. The present invention also relates to a magnetic recording disk device, in brief, a magnetic disk device, for recording and reproducing information, using the magnetic recording medium of the present invention.
2. Description of Related Art
The development of information processing techniques has led to an increasing demand for an increase in the density of magnetic disk devices used in external storage devices for computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, wherein the electric resistance changes in response to the magnetic field intensity, that is, an MR head, instead of the conventional wound-type inductive thin film magnetic head has been proposed in the art. The MR head applies magnetoresistance, that is, the change in electric resistance produced in a magnetic material on application of an external magnetic field, to the reproduction of a signal on a recording medium and has features including a reproduction output margin that is several times larger than that of the conventional inductive thin film magnetic head, a low inductance and a large S/N ratio. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a spin valve GMR head of a practical type, besides the MR head, have also been proposed.
Further, in order to meet the demand for high density recording, a sufficient improvement in properties, to cope with the above MR head, AMR head, or GMR head (including spin valve head) has been demanded of the magnetic recording medium. In particular, low noise is required in the magnetic recording medium, in addition to a high coercive force Hc for high-density recording.
Hitherto, as is well-known in the art, the magnetic recording medium has been generally produced by depositing chrominium on a nonmagnetic substrate such as an aluminum substrate to form an underlayer, followed by depositing a cobalt-based alloy on the resulting chromium underlayer to form a magnetic recording layer.
Further, to obtain a reduced noise level, many changes, such as the addition of an additional element to the alloy of the magnetic recording layer, thereby breaking the magnetic interaction between the magnetic particles or reduction in the particle size of the magnetic particles in the magnetic recording layer, have been made to the magnetic recording medium. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-148411 discloses a low noise and high density recording-type magnetic recording medium which is suitable for use in high density recording devices. The magnetic recording medium disclosed in this publication is characterized in that the Co/Ni-based alloy or Co/Cr-based alloy constituting the magnetic recording layer contains a third element added thereto, that is, any one of Ta, Mo and W or an alloy thereof. Japanese Unexamined Patent Publication (Kokai) No. 7-50008 discloses a magnetic recording medium which can simultaneously satisfy the requirements of a high coercive force and low noise. Specifically, the magnetic recording medium disclosed in this publication comprises a nonmagnetic substrate layer and a magnetic layer deposited through a nonmagnetic metal underlayer (thickness of 10 to 300 nm) of chromium or a chromium alloy on the nonmagnetic substrate layer, the magnetic layer being formed of an alloy containing Co, Cr, Pt, and at least one member selected from the group consisting of Nb, Hf, W, Ti, and Ta. According to this invention, not only a high coercive force of 1,610 to 1,750 Oe (Examples 1 to 7) but also low noise can be achieved. Similarly, Japanese Unexamined Patent Publication (Kokai) No. 7-50009 discloses a magnetic recording medium wherein a thin film medium of an alloy of 95 to 60 at % of Cr and 5 to 40 at % of at least one member selected from Mo and W is used as an underlayer for a single magnetic layer consisting of a CoCrPt alloy. This magnetic recording medium also can realize, simultaneously, a high coercive force and low noise. More specifically, the use of a Cr underlayer containing 28 at % of Mo results in about 10% reduction in noise as compared with the underlayer consisting of Cr alone. In the techniques disclosed in these publications, however, tBr (a product of the layer thickness t and the residual magnetic flux density Br of the magnetic recording layer) is not less than 270 G.xcexcm, rendering these techniques unsatisfactory for the higher density recording expected in the future.
Furthermore, recently, since the magnetic disk devices are carried for business and other uses, a glass substrate is frequently used in place of the aluminum substrate to improve a resistance to shock of the devices. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-197941 discloses a thin metallic layer-type magnetic recording medium characterized by depositing, on a nonmagnetic glass substrate, a nonmagnetic heat storage layer having good heat conductivity (Cr, Ti or CrTi alloy) and a NiP layer in the described order, followed by forming a magnetic recording layer of Co alloy through an applied Cr underlayer. According to this invention, since a nonmagnetic heat storage layer having good heat conductivity is deposited at a large thickness of 300 to 1500 xc3x85 under the-NiP layer, a crystallization of NiP due to sudden increase of the temperature of the NiP layer during heating of the same layer at a temperature of 250 to 300xc2x0 C. on a heating device such as an IR heater can be prevented and, as a function thereof, an undesirable reduction of the coercive force of the Co alloy can also be prevented. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 5-314471 discloses a substrate for use in a magnetic recording medium characterized by having a nonmagnetic NiP layer having a surface roughness, in terms of a maximum height (R max), of 500 xc3x85 or less, produced upon electroless plating, on a glass substrate having a surface roughness, in terms of a maximum height (R max), of 500 xc3x85 or less. According to this invention, a magnetic recording medium having excellent durability due to use of the glass substrate and excellent corrosion resistance due to application of the electroless plating layer can be provided.
In this connection, the inventors have measured the read/write performances for the above-discussed magnetic recording media and other conventional magnetic recording media using a glass substrate as the nonmagnetic substrate, and found that sufficiently high read/write performances cannot be attained when the recording density is the level required at present, that is, generally 1 Gb/in2 or more. In other words, for these magnetic recording media, it becomes possible to obtain low noise but it is difficult to avoid drawbacks which are caused simultaneously, such as a reduction of reproducing output. It is therefore desirable to ensure a remarkable reduction of noise in the media, i.e., highly increased S/N ratio, while maintaining a high reproducing output.
The related prior art will be further described in connection with the above descriptions. In the magnetic recording medium, a nonmagnetic underlayer to be sandwiched between the nonmagnetic substrate and the magnetic recording layer is generally constituted from chromium or an alloy thereof. This is because, if the chromium-based material is used as the underlayer material, it can adjust a direction of easy magnetization in the overlying magnetic recording layer consisting of a cobalt-based alloy to an areal direction. In fact, it is well-known that the underlayer is preferably formed from chromium alone or an alloy of two or more components containing chromium as a principal component thereof. However, the inventors have found that for the prior art magnetic recording media using a glass substrate, the conventional chrominum-based underlayer used therein can cause an unusual increase of noise which means that a good SIN ratio cannot be obtained.
In addition, it is also well-known that a so-called xe2x80x9ctexturexe2x80x9d or xe2x80x9ctexturingxe2x80x9d may be applied to an aluminum substrate provided with a nonmagnetic NiP plating surface which is widely used in the production of prior art magnetic recording media to further orient a direction of easy magnetization of the magnetic layer to a circumferential direction, thereby increasing a S/N ratio of the medium. However, the experiments by the inventors have indicated that the texturing in a surface of the glass substrate does not create a magnetic anisotropy in the circumferential direction, contrary to the above-described texturing in the aluminum substrate.
Also, it is well-known that the texturing is effective to reduce an adsorption of the head onto the magnetic recording medium. However, in the prior art, the application of texturing to a surface of the glass substrate has not been practiced due to its low processability. As an alternative, there is a well-known method to laminate an irregular film, called a film texture, onto a surface of the glass substrate. However, the lamination of the film texture can deteriorate an areal orientation of the magnetic layer. Other texturing methods are also known to cause an increase of the grain size in the magnetic layer, thus lowering a S/N ratio of the medium.
In addition to the prevention of adsorption between the recording medium and the recording head, the texturing is also effective to reduce frictional force on a surface of the medium, because fine irregularities (depressions and projections) can be given to the medium surface. That is, in the magnetic recording disk device based on a CSS system, the recording head and the recording medium are in close contact with each other when the device is stopped, and then, when the device is again operated, a certain level of frictional force is generated between the head and the medium. Such frictional force can be reduced due to presence of the irregularities in the medium surface.
As described above, recently, there is a tendency to use a hard glass substrate in place of the aluminum substrate to satisfy the requirements concerning a high resistance to shock or impact, because the medium is generally used under the severe conditions in portable terminals of the magnetic disk device and the device itself has high performances which can be deteriorated by incorrect handling of the device. And, in view of the difficulty in the fabrication of fine irregularities in the glass substrate, methods other than the above-described film texture method have been adopted in the production of magnetic recording medium. Typical examples of the irregularity-providing methods are:
tape texturing in which a substrate is rotated with application of an alumina grain-coated polishing tape to produce circumferential grooves in a surface of the substrate;
slurry texturing in which a pad with the impregnated polishing solution containing alumina or diamond powders is pressed against a substrate to produce circumferential grooves in a surface thereof;
application of an irregularity-providing layer made from aluminum, titanium and the like to a substrate surface; and
NiP plating on an aluminum substrate, thereby increasing a strength of the substrate.
However, the last method, i.e., NiP plating, is unsatisfactory, because as previously mentioned, the magnetic recording medium with the NiP-plated aluminum substrate does not ensure a high impact resistance necessary for high performance devices which will be developed in the future. Further, even if a hard substrate is used in view of obtaining a high impact resistance, the first or second method, i.e., tape texturing or slurry texturing, is insufficient to obtain a satisfactory surface roughness because of the low processability of the hard substrate. Thus, for the hard substrate, the third method, i.e., application of an irregularities-providing layer, is principally utilized. However, the resulting medium cannot be applied for use in combination with a SFS (stiction-free slider) head due to unacceptably large wearing amount of the pad of the SFS head. Therefore, it is required to develop both a new recording head and a new recording medium which can satisfy the requirement of a low recording head flying height of not more than 25 nm. Apparently, all of the above-described problems cannot be neglected in the production of magnetic disk devices having a high reliability. Accordingly, although a hard substrate shows poor processability, it is desired to enable use of such a hard substrate, with the advantages that the conventional production process can be used to form uniform irregularities on a substrate surface, thereby avoiding any problems with the low flying recording head, and also the impact resistance of the medium can be improved.
The objects of the present invention are directed to solving the many problems which were described above with reference to the prior art magnetic recording media and devices.
A first object of the present invention is therefore to provide a magnetic recording medium, particularly a longitudinal magnetic recording medium, which has a nonmagnetic substrate consisting of glass or silicon and having a surface with irregularities sufficient to show a low adsorptivity, and which exhibits a high coercive force thereby ensuring a high reproducing output, and inhibits an increase in noise or, preferably, reduces noise, and also which can improve a resolution of the reproduced output and the S/N ratio.
In the magnetic recording medium as the target of the present invention, one of the resulting properties is defined referring to a S/N ratio of the medium when the medium is used at the recording density of 1 Gb/in2 or more, and the S/N ratio should be at least 20 dB.
A second object of the present invention is to provide a magnetic recording disk device using the specific magnetic recording medium according to the present invention.
The above-described and other objects of the present invention will be easily understood from the following detailed description of the present invention.
In one aspect thereof, the present invention provides a magnetic recording medium comprising a nonmagnetic glass or silicon substrate having non-oriented irregularities on a surface thereof, and, having applied thereon in the following order:
an underlayer which comprises a second underlayer consisting of nickel (Ni) and phosphorus (P) and a third underlayer containing chromium (Cr) as a principal component thereof which are formed in the described order, in the presence or absence of a first underlayer containing chromium (Cr) as a principal component thereof, on said substrate, and
a magnetic recording layer which has a circumferential direction of easy magnetization and contains cobalt (Co) as a principal component thereof, and also contains chromium (Cr) and platinum (Pt) in combination with tantalum (Ta) or tantalum (Ta) and niobium (Nb).
In the magnetic recording medium of the present invention, the underlayer between the nonmagnetic substrate and the magnetic recording layer may be either a two layer structure consisting of the second NiP underlayer and the third Cr-based underlayer, or a three layer structure consisting of the first Cr-based underlayer, the second NiP underlayer and the third Cr-based underlayer.
According to the present invention, because of its specific layer structure mentioned above, a magnetic recording medium having no adsorption problem and showing remarkably reduced noise can be provided. Further, contrary to the prior art which texturing in a surface of the glass or silicon substrate was not practically carried out due to poor processability, according to the present invention, such texturing can be advantageously applied to the glass or silicon substrate, as in application to other conventional substrates, because a NiP layer is applied over a surface of the substrate. Furthermore, according to the present invention, a magnetic recording medium showing a highly increased S/N ratio can be provided, if the magnetic recording layer is constituted from the specific-four-component alloy of Co, Cr, Pt and Ta or the specific five-component alloy of Co, Cr, Pt, Ta and Nb.
In addition, the inventors have found that in the lamination of the NiP layer over a surface of the glass or silicon substrate, the formation of a Cr-based layer over the substrate surface, followed by lamination of the NiP layer is particularly suited to the practice of the present invention. Described in detail, when a Cr layer (underlayer) and a CoCr-based magnetic recording layer are directly applied in the described order over the substrate such as glass or silicon substrate, a poor longitudinal or areal orientation of C-axis is observed in a hcp phase of the Co crystal in the magnetic layer, if a Cr concentration in the layer is 15 at % or more. An increase of the thickness of the Cr layer is effective to improve the longitudinal orientation, however, it causes an increase of the grain size of the Cr layer and thus the overlying CoCr-based magnetic layer. An increase in the noise in the medium is thus caused as a result of the increased grain size in these two layers. The inventors have studied the above problem, and have now found that the formation of coarse grains in the magnetic layer can be effectively prevented and thus an increase of the noise can be prevented, if a sputtered NiP layer is sandwiched between the substrate and the Cr layer. The inventors have also found that in the application of the NiP layer, the above-described order of the steps, i.e., formation of the Cr layer, followed by formation of the NiP layer, is effective to increase an adhesion between the substrate surface and the NiP layer.
In a preferred embodiment of the present invention, the present invention resides in a magnetic recording medium comprising a nonmagnetic substrate having applied thereon, through an underlayer, a magnetic recording layer, in which the underlayer comprises a second underlayer of the nickel alloy which is formed with sputtering on the substrate and is subjected to texturing. Preferably, the second underlayer further comprises a third chromium-based underlayer thereon.
In another aspect thereof, the present invention provides a magnetic recording disk device comprising a recording head section for recording information in a magnetic recording medium and a reproducing head section for reproducing information, wherein the magnetic recording medium is the magnetic recording medium of the present invention described above and described below in detail and the reproducing head section is provided with a magnetoresistive head.
The magnetoresistive head used in the magnetic disk device of the present invention preferably includes the MR head, the AMR head, the GMR head and the spin valve GMR head, because these heads can be operated with high performances. Further, the magnetoresistive head is preferably disposed on a stiction-free slider (SFS) which contains rails for creating a flying force wherein two or more projections or protrusions are disposed on a flying surface of the rails to prevent stiction. In other words, in the practice of the present invention, the magnetic recording medium of the present invention is preferably used in combination with the flying type magnetoresistive head provided with a slider containing stiction-preventing protrusions on a flying surface of the appended rails.
As will be appreciated from the following detailed description of the present invention, according to the present invention, the magnetic recording medium having a higher reproducing output and lower noise in comparison with the prior art magnetic recording media can be provided. Thus, the present invention ensures a higher S/N ratio at the high density recording, thereby providing a magnetic recording disk device for high density recording which is better than the prior art disk devices.
In addition to the above advantages, according to the present invention, an undesirable increase in noise can be prevented and, preferably, noise can be remarkably reduced, while enabling use of a hard substrate such as glass or silicon substrate without causing any texturing and other problems.